While lithography is utilized in the fabrication of semiconductor integrated circuits having an ever increasing degree of integration, the exposure system is in advancement toward a shorter wavelength. Progressive shifts have been made from the ultraviolet radiation including g-line (436 nm) and i-line (365 nm) to deep ultraviolet radiation, typically KrF excimer laser (248 nm) and ArF excimer laser (193 nm) and even to F2 laser (157 nm).
The reduction of exposure wavelength improves the resolution, but reduces the depth of focus (DOF), which leads to a narrower process margin and lower stability, imposing detrimental impact on the manufacturing yield of products.
One approach for overcoming the problem is a phase shift method. Inter alia, the use of halftone phase shift masks improves DOF. Since an increase of DOF enlarges the process margin, this is indispensable for the future microprocessing technology. The future lithography candidates proposed by the International Technology Roadmap for Semiconductors (ITRS) include a phase shift mask (PSM) adapted to exposure to ArF excimer laser as a candidate for the technology node 90 nm.
The phase shift mask has a phase shifter film through which exposure light is phase shifted 180 degrees. The light transmitted by the phase shifter film pattern and the light transmitted by the portion where the phase shifter film is absent have reverse phases. These lights of reverse phases overlap at the interface therebetween so that the light intensity becomes zero, yielding a light intensity distribution where an acute change appears at the interface. This results in an increased DOF and an improved image contrast.
The phase shift masks include Levenson, halftone and other types. Of the halftone phase shift masks proposed thus far, single layer halftone phase shift masks have a simple structure. The single layer halftone phase shift masks proposed thus far have phase shifter films made of amorphous silicon, silicon nitride, molybdenum silicide oxide (MoSiO) or molybdenum silicide oxynitride (MoSiON) as described in JP-A 7-140635.
In preparing these phase shift masks, a method of patterning a phase shift mask blank by lithography is typically employed. The lithography process involves applying a resist onto a phase shift mask blank, irradiating selected portions of the resist coating with electron beams or ultraviolet radiation, developing the resist coating until the phase shifter film surface is exposed in the irradiated portions, etching the phase shifter film through the patterned resist coating as a mask until the substrate is exposed, thereafter stripping the resist coating. A phase shift mask is obtained in this way.
In most photomask blanks as typified by phase shift mask blanks, a film such as phase shifter film is generally deposited on a substrate by sputtering. Stress is introduced into the film by which the underlying substrate is more or less distorted. The resulting photomask blank thus suffers warp due to the film stress. When a photomask is prepared by pattering such a warped photomask blank whereby the film is partially removed as a result of patterning, the film stress applied to the substrate is partially released so that the substrate is recovered from the warped state near to the state prior to the deposition. As a result, the substrate changes its flatness. This change introduces a positional shift between the stage of pattern exposure and the actually finished mask, which provides a greater influence as the mask pattern becomes finer. It also causes a focusing offset when pattern exposure is carried out using the photomask.